Numerous prosthetic joints have been developed for replacing joints in human bodies, including joints for the toes and fingers. Many of these joints involve a ball-and-socket connection wherein two joint parts are physically held or joined together in a swiveling relationship, with such joints requiring the two parts to be physically snapped or joined together during the implanting operation. Such connection, however, can be difficult to perform, particularly when replacing small joints such as toe joints.
In an effort to provide a small joint which does not require such connection, various implantable joints have been proposed wherein the two joint parts have opposed head and socket parts which provide a swivel or swingable connection, but which do not require a physical joining of the two parts together. Most of these prior attempts, however, have proven less than desirable for various reasons. For example, some of the known joints have not provided proper mating and swiveling relationship between the joint parts in order to accurately reproduce the necessary natural swiveling movement of the joint being replaced. Others of these known joints have provided structural or geometrical features which have been less than optimum with respect to either surgically implanting the joint and/or replacing the natural joint structure and its inherent function.
Examples of prior art joints are illustrated by U.S. Pat. Nos. 4,156,296, 4,231,121, 4,367,562, 4,242,759 and 3,946,445.
Accordingly, it is an object of this invention to provide an improved prosthesis joint, specifically an implantable joint for replacing the human toe joint, which implantable joint is for connection between the metatarsal and middle phalangeal bones and provides a functional cooperation which closely depicts the desired and permissible movements of the natural joint.
In the improved implantable toe joint of this invention, there are provided two one-piece members, namely a tack member of metal which is implanted into the end of the metatarsal, and a socket member of plastics which is implanted into the adjacent end of the middle or proximal phalange. The tack member has a smooth convex part-spherical bearing surface which is adapted to freely slidably contact a smooth concave part-spherical bearing surface on the socket member, these two bearing surfaces being generated about the same radius. The convex bearing surface on the tack member extends symmetrically sidewardly in opposite directions through an angle which approaches but is less than 180.degree., and this convex bearing surface extends upwardly through an angle less than 90.degree. but downwardly through an angle substantially in excess of 90.degree. so that the tack member has an enlarged head portion which defines thereon the bearing surface. This head portion has a lower rearwardly projecting extension to provide smooth arcuate sliding between the joint members over a permissible range while providing positive contact support between the joint members throughout the swiveling range. The tack member has a substantially cylindrical stem projecting rearwardly from the head portion for implantation into the metatarsal, this stem being of a decreasing tapered cross section as it projects rearwardly. The longitudinal axis of the stem is positioned in downwardly spaced but substantially parallel relationship relative to the longitudinal axis of the head portion, with the former longitudinal axis being substantially concentrically aligned with the longitudinal axis of the metatarsal. The socket member has a head part which defines a concave bearing surface thereon, this head part being of a substantially horizontally elongated transverse cross section so that the concave bearing surface has an arcuate extent in the vertical direction which is rather small in relationship to the vertical arcuate extent of the convex bearing surface so as to permit substantial vertical swiveling between the tack and socket members when implanted. The head part of the socket member, and more specifically the concave bearing surface, extends sidewardly or horizontally through an arcuate extent which is similar to but preferably slightly smaller than the horizontal arcuate extent of the convex bearing surface. The head part of the socket member has a stem portion which projects outwardly for implantation into the phalange. The stem portion has a plurality of axially spaced annular ribs or flanges associated therewith, these flanges being resiliently deflectable to effect a self-holding or locking of the socket member to the phalange when implanted thereon.
Other objects and purposes of the invention will be apparent to persons familiar with joints of this general type upon reading the following specification and inspecting the accompanying drawings.